The goal of this project is to determine the feasability of xenogeneic pig thymus to engraft in an SIV infected macaque, and to determine if this has any impact on T cell reconstitution The initial aim was to establish baseline assays to determine the degree of immunosupression as a result of SIV We have conducted comprehensive analysis to determine the optimal in vitro conditions to determine both alloantigen and xenoantigen responses in macaques Our data demonstrate poor to absent xeno and allo- responses in macaques with advanced SIV disease, as well as 1-2 log reduction in the proliferation response to lectin Based on the poor in vitro proliferative responses we wished to examine whether pig thymic tissue might engraft in SIV-infected macaques without any conditioning We therefore performed transplants of pig thymic tissue in 2 SIV-infected macaques with advanced disease (CD4<200 mm3) Biopsy of the transplant site at 30 days revealed lymphoid aggrega tes of rhesus T cells in both animals, with only minimal inflammatory changes outside the lymphoid aggregates No evidence for engraftment of pig thymic tissue was observed (negative results for pig cytokeratin, MHC class I, MHC class II, and CD2) These results suggest that even SIV-infected animals with advanced disease are able to mount sufficient immune responses to reject swine thymus Due to our inability to achieve engraftment of pig thymus in SIV infected macaques in the absence of conditioning (see below), we altered our experimental design to include a conditioning protocol, namely cyclophosphamide anti-thymocyte globulin (ATG) and cyclosporine In the first animal examined, we achieved 99% depletion of peripheral T cells (CD3+) following ATG treatment, but the CD3+ count rapidly returned to baseline within 3 days The second animal received 3 additional days of ATG, but we observed poor T cell depletion only (90%) depletion after 6 days of ATG Both animals received pig thymus in both the omentum and quadriceps Antiretroviral therapy as well as PCP prophylaxis was continued Biopsies of the quadriceps muscle transplant sites at 1 and 3 months demonstrated profound infiltration of rhesus CD3+CD8+ T cells into the area, with no evidence for engraftment as detected by immunohistochemistry for cytokeratin as well as pig-specific antigens We concluded that we had a low level engraftment, but no clear functional thymus was detected A clear problem in data interpretation is that the animals both had detectable thymus in the mediastinum at autopsy, so the rebound of naive T cells cannot be used as an indicator of engraftment and function, this will be addressed by performing thymectomy prior to SIV-infection in subsequent animals